Decorative coating materials, also referred to as decorative papers or decorative foils are primarily used as a surface coating in furniture manufacturing and interior fittings, particularly laminate floors. Decorative paper/decorative foil is understood to be printed or unprinted papers that have either been impregnated with synthetic resin or impregnated with synthetic resin and undergone surface treatment. Decorative papers/decorative foils are glue-bonded or adhesive-bonded to a backing panel.
Depending on the type of impregnation operation, a distinction is made between decorative papers/decorative foils with a thoroughly impregnated paper core, and “prepregs”, in which the paper is only partially impregnated online or offline in the paper machine None of the previously known prepregs, which contain formaldehyde-containing duroplastic resins or acrylate-containing binders that are low in formaldehyde, satisfies all of the requirements placed on it, such as good plybond strength and good adhesion after it has been painted and stuck to a wood-based sheet material board.
In order to bond the decorative foils to wood materials such as chipboard or MDF board, the adhesives normally used are urea-based glues or polyvinyl acetate (PVAC) glues. These do not always guarantee that the decorative foils will be bonded properly.
High pressure laminates are laminates that are produced by compressing a number of impregnated, stacked papers against each other. The structure of these laminates generally includes an uppermost, transparent covering sheet (the overlay), which provides high surface resistance, a resin-impregnated decorative paper, and one or more kraft paper sheets impregnated with phenolic resin. The base (substratum) may be formed by hardboard and chipboard panels or plywood for example.
In the laminates produced according to the short cycle process (low pressure laminates), the decorative paper soaked in resin is pressed directly against a base, for example a chipboard, with the application of low pressure.
In the processing industry, very high demands are placed on the bondability and adhesion of the glued decorative foil. For example, adhesion must be good immediately after the gluing process, in order to prevent the freshly laminated panel from being damaged by further handling. The panels are often machined further just a few minutes to hours after the decorative foil is glued on, in sawing, milling and drilling processes, and the applied decorative foil must not separate or tear at the machined edges. The finished surfaces are often packaged for further transport as well, and adhesive tapes are used for this, being affixed directly to the decorative surface. These adhesive tapes must have sufficient adhesive strength, but they must be removable without residue and without damaging the decorative foil to which they stick after transporting operations are complete. The decorative foil must therefore have high plybond strength perpendicularly to the decorative surface after it has been glued as well.
The decorative paper used in the coating materials described in the preceding is used in the white or coloured state and with or without additional printing.
With regard to their technical application properties the decorative base papers that are used as the starting materials must satisfy certain requirements. These include high opacity for better coverage of the base, uniform formation and grammage of the sheet for homogeneous resin absorption, a high degree of resistance to light, high purity and colour evenness for good reproducibility of the pattern to be printed, wet strength to ease the impregnation process, corresponding absorbency to achieve the required degree of resin saturation, dry strength, which is important in winding operations in the paper machine and during printing in the printing machine.
In order to achieve a decorative surface, the decorative base papers may be printed. Printing is mostly done by the rotogravure printing process, in which the printed image is transferred to the paper by means of several gravure rollers. The individual printed dots are to be transferred completely and as intensively as possible to the surface of the paper. But it is precisely in the decorative gravure printing that sometimes only a fraction of the raster points present on the gravure rollers is transferred to surface of the paper. “Missing dots”, this is to say voids, occur. The printing colour often penetrates too deep into the paper structure, which in turn reduces the colour intensity. The prerequisites for a good printed image with few voids and high colour intensity are thus a paper surface topography that is as smooth as possible and balanced colour acceptance behaviour of the paper surface.
For this reason, base papers are usually smoothed with soft calenders, and in some cases also with Janus calenders. This treatment can cause the paper surface to become bruised and consequently compacted, which impairs its resin absorption capability.
The properties described in the preceding are influenced significantly by the impregnation of the decorative base paper, that is to say, by the nature of the impregnation medium used.
The impregnation resin solutions normally used for impregnating the decorative base papers are resins based on urea, melamine, or phenolic resins and containing formaldehyde, and result in brittle products with poor tear propagation resistance and printability.
In recent times, it has become more and more important to ensure that the impregnation resin solutions used for impregnating decorative base papers are free from substances that may be harmful to human health, particularly that they contain no formaldehyde. Furthermore, the components used should originate from renewable raw materials to the extent possible.
The use of a formaldehyde-free resins with a base of an acrylic acid ester styrene copolymer to produce non-yellowing prepregs is described in DE 197 28 250 A1. The disadvantage of this material is that it produces a product with poor resistance to splitting and inadequate adhesion strength after bonding.
Formaldehyde-free impregnation resin solutions for impregnating decorative base papers are also described in EP 09 648 248 A1 and EP 0 739 435 A1. These preferably consist of a styrene acrylic acid copolymer and polyvinyl alcohol. Unfortunately, the paper that is impregnated with such an impregnation resin solution is also in need of further improvement in terms of its plybond strength and adhesion after bonding.
In WO 2001/11139 A1, a formaldehyde-free compound consisting of a binding agent, an aqueous polymer dispersion and glyoxal is suggested, with which decorative papers that are highly resistant to splitting can be produced. However, the paper impregnated with this compound does not bond well.
In WO 2009/000769 A1, a formaldehyde-free compound consisting of a styrene-acrylic acid ester copolymer and a starch having a particular molecular weight distribution is described. However, the properties of this prepreg with regard to bonding after adhesion still need improvement.